Microelectromechanical Systems (MEMS) devices are widely used in applications such as automotive, inertial guidance systems, household appliances, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems. Such MEMS devices are used to sense a physical condition such as acceleration, pressure, or temperature, and to provide an electrical signal representative of the sensed physical condition.
Capacitive-sensing MEMS designs are highly desirable for operation in high acceleration environments and in miniaturized devices, due to their small size and suitability for low cost mass production. Conventional MEMS capacitive sensors operate so that a flexibly mounted seismic mass, also known as a proof mass, is deflectable in at least one direction by a property being sensed, e.g., acceleration. Deflection of the proof mass causes a change in capacitance of a differential circuit that is connected to it. This change in capacitance is a measure of the property being sensed.
FIG. 1 shows a side view of a portion of a prior art MEMS device 20. In this example, MEMS device 20 a two layer capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable proof mass 22 or plate that rotates under z-axis acceleration, represented by an arrow 24, above a substrate 26. This rotation occurs because an axis of rotation 28 is offset such that one end of proof mass 22 is heavier than the other end. The accelerometer structure can measure two distinct capacitances, represented by C1(SIG) and C2(SIG), between proof mass 22 and two sense plates 30 and 32 that are symmetrically located relative to axis of rotation 28 in order to determine differential or relative capacitance. A gap 34 is formed between proof mass 22 and each of sense plates 30 and 32 to provide space for the rotation of proof mass 22 about axis of rotation 28 and subsequent measurement of capacitances indicative of z-axis acceleration 24. In the illustrated embodiment, gap 34 is sometimes referred to as a “z gap” because gap 34 is formed between layers of structural material during processing and is thus in an out of plane direction relative to substrate 26.